1. Field of the Invention
The present invention relates to coherent detection and, more specifically, to a low cost, compact, and temperature-insensitive optical hybrid.
2. Description of the Related Art
Since the late 1990s, the transport capacities of ultra-long-haul and long-haul fiber-optic communication systems have been significantly increased by the introduction of erbium-doped fiber amplifier (EDFA) technology, dense wavelength division multiplexing (DWDM), dispersion compensation, and forward error correction (FEC) technologies. For fiber-optic communication systems utilizing such technologies, the universal on/off-keying (OOK) modulation format in conjunction with direct detection methods has been sufficient to address data rates up to 10 Gb/s per channel.
In order to economically extend the reach and data capacity beyond such legacy systems and into next-generation networks, several technological advancements must take place, including, without limitation, the adoption of a differential phase-shift keying (DPSK) modulation format, as opposed to OOK; further developments in optical coherent detection; and progress in adaptive electrical equalization technology. In combination, these improvements would materially improve signal robustness and spectral efficiency against noise and transmission impairments.
Such crucial advances in optical signal technology are no longer theoretical possibilities but actually represent feasible objectives in present-day optical networking technology. The path for an optical coherent system has already been paved through the deployment of DPSK modulated systems by Tier-1 network providers and the increased computational capacity and speed of electronic DSP circuits in receivers, which provides an efficient adaptive electrical equalization solution to the costly and difficult optical phase-lock loop. These advances coupled with a commercially feasible optical hybrid solution are likely to cause Tier-1 providers and carriers to reassess their rationales for not adopting and implementing an optical coherent detection scheme. Perhaps the use of coherent detection in optical networks will enable the same benefits already realized in microwave and RF transmission systems in relation to extended capacity and increased transmission distance without repeaters.
The commercial feasibility of a coherent system for optical signal transmission was first investigated around 1990 as a means of improving a receiver's sensitivity. In contrast with existing optical direct-detection system technology, the optical coherent detection scheme would detect not only the optical signal's amplitude, but phase and polarization as well. With the optical coherent detection system's increased detection capability and spectral efficiency, more data could be transmitted within the same optical bandwidth. Moreover, because coherent detection allowed the optical signal's phase and polarization to be detected, and therefore measured and processed, transmission impairments that previously presented challenges to accurate data reception could in theory be mitigated electronically when the optical signal was converted into an electronic signal. However, the technology never gained commercial traction because the benefits of an optical coherent system could not be implemented with existing systems and technologies.
The implementation of a coherent detection system in optical networks requires the ability to stabilize frequency differences between transmitter and receiver within close tolerances, the capability to minimize or mitigate frequency chirp or other signal inhibiting noise, and the availability of an “optical mixer” to properly combine the signal and the local amplifying light source or local oscillator (LO). These technologies were not available in the 1990s. A further setback to the adoption and commercialization of an optical coherent system was the introduction of the EDFA, an alternative low cost solution to the sensitivity issue.
Notwithstanding this myriad of challenges, an optical coherent system (also referred to as “Coherent Light Wave”) remains a holy grail of sorts to the optical community because of its advantages over traditional detection technologies. Coherent Light Wave provides an increase in receiver sensitivity in the order of 15 to 20 dB compared to incoherent systems, thereby permitting longer transmission distances (up to an additional 100 km at a wavelength of about 1.55 μm in fiber). This enhancement is particularly significant for space-based laser communications where a fiber-based solution similar to the EDFA is not available. It is compatible with complex modulation formats, such as DPSK or DQPSK. Moreover, concurrent detection of a light signal's amplitude, phase and polarization allows more detailed information to be conveyed and extracted, thereby increasing tolerance to network impairments, such as chromatic dispersion, and improving system performance. Better rejection of interference from adjacent channels in DWDM systems also allows more channels to be packed within the transmission band. Linear transformation of the received optical signal to an electrical signal enables analysis using modern DSP technology and it is suitable for secured communications.
There is a growing economic and technical rationale for the adoption of a coherent optical system at present. Six-port hybrid devices have been used for microwave and millimeter-wave detection systems since the mid-1990s and are a key component for coherent receivers. In principle, the six-port device consists of linear dividers and combiners interconnected in such a way that four different vectorial additions of a reference signal (LO) and the signal to be detected can be obtained. The levels of the four output signals are detected by balanced receivers. Thus, by applying suitable base-band signal processing algorithms, the amplitude and phase of the unknown signal can be determined.
For optical coherent detection, a six-port 90-degree optical hybrid should mix the incoming signal with the four quadratural states associated with the reference signal in the complex-field space. The optical hybrid should then deliver the four light signals to two pairs of balanced detectors. Let S(t) and L denote the two inputs to the optical hybrid and S(t)+Lexp[j(π/2n)], with n=0, 1, 2 and 3, represent the four outputs from it. Using the PSK modulation and phase-diversity homodyne receiver as an illustration, one can write the following expression for the signal power to be received by the four detectors:Pn(t)∝PS+PL+2√{square root over (PSPL)} cos [θS(t)+θC(t)−π/2n], n=0, . . . 3;  (1)where PS and PL are the signal and reference power, respectively, θS(t) is the signal phase modulation, and θC(t) is the carrier phase relative to the LO phase. With proper subtractions, the two photocurrents fed to the TIAs can be expressed asIBD1∝√{square root over (PSPL)} cos [θS(t)+θC(t)], and  (2)IBD2∝√{square root over (PSPL)} sin [θS(t)+θC(t)],  (3)which encompass both the amplitude and phase information of the optical signal. Accordingly, the average electrical signal power is amplified by a factor of 4PL/PS. Following this linear transformation, the signals are electronically filtered, amplified, digitized and then processed. Compared to a two-port optical hybrid, the additional two outputs have eliminated the intensity fluctuation from the reference source (LO).
An optical coherent receiver requires that the polarization state of the signal and reference beam be the same. This is not a gating item as various schemes or equipment are available to decompose and control the polarization state of the beams before they enter the optical hybrid. Further, certain polarization controllers can be used to provide additional security functionality for optical coherent systems, preventing third parties from tapping information or data streams by implementing polarization scrambling and coding techniques.
For laboratory purposes, a 90-degree optical hybrid has traditionally been constructed using two 50/50 beam splitters and two beam combiners, plus one 90-degree phase shifter. These optical hybrids can be implemented using all-fiber or planar waveguide technologies; however, both methods have their respective drawbacks. Both technologies require sophisticated temperature control circuits to sustain precise optical path-length difference in order to maintain an accurate optical phase at the outputs. In addition, fiber-based devices are inherently bulky and are unstable with respect to mechanical shock and vibration. Waveguide-based products suffer from high insertion loss, high polarization dependence and manufacturing yield issues. Waveguide-based products are also not flexible for customization and require substantial capital resources to set up.
Accordingly, a low-cost, temperature insensitive and vibration/shock resistant optical hybrid and a method of operating the same are very desirable at this time and are provided by the present invention.